This invention relates to an interferometer and more particularly to a compact, portable diffraction moire interferometer useful for determining material distortion and strain.
There are numerous techniques available to determine some aspect of material distortion or strain, including holographic interferometry, speckle interferometry, photoelastic effects and strain gauges. Each of these methods has been embodied in a device which packages the technique and allows for repeated use under controlled conditions, usually by an operator not thoroughly familiar with why or how the device works.
With this invention, Applicant provides a compact, portable device, easily used by an inexperienced operator, utilizing the technique of diffraction moire interferometry. For the purpose of determining material distortion diffraction moire interferometry provides advantages not shared in total by any of the above methods. The technique of diffraction moire interferometry and diffraction grating interferometry, upon which it is based, is directly sensitive to in-plane displacements (unlike holography, photoelasticity and strain gauges), provides full field data over a substantial area (unlike strain gauges), provides excellent, variable resolution, and, finally, provides very high quality data, not degraded by laser speckle (as with holographic and speckle interferometry).
The term interferometer may be applied to any arrangement whereby a beam of light is separated into two or more parts, the parts being subsequently reunited after traversing different optical paths. Diffraction moire interferometry uses a reflective-type diffraction grating (i.e. a specimen grating) which is fixed to the object under study and illuminated by two mutually coherent collimated beams. If the illuminating beams are set at the proper incidence angles, the plus first diffraction order of one illuminating beam and the minus first diffraction order of the other beam coincide in space along a line normal to the specimen.
As a result, interference fringes (sometimes called moire patterns) representing a contour map of in-plane displacements can be observed. Comparison of fringes before and after loading can be used to determine load-induced displacements.
Sensitivity of the technique is dependent upon the frequency of the diffraction grating placed on the specimen and on the wavelength used to illuminate the grating. Sensitivities of better than 0.5 micrometer are routinely obtained, and various interpolations can further extend this resolution.
Until now, set-up for moire interferometer measurement has been ad hoc. The standard method requires complex and very clumsy optical set-ups, including lasers, spatial filters, collimators, beamsplitters, mirrors and path-matching arrangements. The procedure often requires several hours for a very experienced operator and is formidable indeed for the inexperienced.
Portable instruments have incorporated a method of mechanical interference which also employs moire patterns but this method, called moire deflectometry or coarse moire, is distinguishable from diffraction moire interferometry (see U.S. Pat. No. 4,577,940 Krasinski et al.) Coarse moire are produced by superpositioning two coarse amplitude gratings in intimate contact. As one grating is distorted, areas of light and dark appear, due to the mechanical obstruction of light by the opaque bars of the gratings. Sensitivity is increased by using gratings of fine pitch and high spatial frequency (see further, Daniel Post, "Optical Interference for Deformation Measurements," Mechanics of Nondestructive Testing, 1980.)
The so-called moire patterns of diffraction moire interferometry are sometimes explained as the result of the same kind of mechanical obstruction, between the specimen grating and what is termed a "virtual reference grating"--the array of fringes, and the walls of interference from which they are derived. While there is no physical reality to this explanation, it is sometimes convenient conceptually (see Daniel Post, "Moire Interferometry", Handbook on Experimental Mechanics, 1987, p. 333).
Moire deflectometry has been preferred to diffraction moire interferometry because it involves very simple alignment and therefore is much easier and less expensive to set up (see for example, U.S. Pat. No. 4,459,027, Kafri et al., and U.S. Pat. No. 4,577,940, Krasinski et al.). However, coarse moire does not provide adequate resolution for many modern applications.
It is an object of this invention to provide an improved apparatus for the measurement of deformed gratings.
It is another object of this invention to provide an improved apparatus for determination of material distortion or strain.
It is another object of this invention to provide a diffraction moire interferometer which is compact, portable, convenient to use, and requires little user familiarity with the underlying concepts.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.